The present invention relates to pressure transmitters. More specifically, the invention relates to pressure transmitters utilizing an isolating diaphragm and fill fluid to transmit a process pressure to a pressure sensor.
A process transmitter generally includes a transducer or sensor that responds to a process variable. A process variable generally refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH and other properties. Pressure is considered to be a basic process variable in that it can be used to measure flow, level and even temperature.
Pressure transmitters are commonly used in industrial processes to measure and monitor pressures of various industrial process fluids, such as slurries, liquids, vapors and gases of chemicals, pulp, petroleum, gas, pharmaceuticals, food and other fluid-type processing plants. Often times pressure transmitters are subject to certain industrial applications that are harsh and of varying environmental conditions. For example, a pressure transmitter can undergo extreme temperature changes.
Transmitters that can sense pressure typically include a pressure sensor coupled to at least one isolation diaphragm. The isolation diaphragm is positioned in an opening of the transmitter and isolates the pressure sensor from the harsh process fluids being sensed. Pressure is transferred from the process fluid to the pressure sensor through a substantially incompressible fill fluid carried in a passageway extending from the isolation diaphragm to the pressure sensor.
Some pressure transmitters are components of remote seal systems. In a remote seal system, an isolation diaphragm is remotely located from the pressure transmitter because of the corrosive nature or temperature of the industrial process fluid being sensed. The remotely located isolation diaphragm is fluidically coupled to the pressure sensor by a fill fluid that fills a capillary tube and cavities of a remote seal body which houses the remotely located isolation diaphragm. The fill fluid conveys the exerted pressure of the process fluid through the capillary tube to the pressure sensor in the pressure transmitter.
The fill fluid used in either of the above-described pressure sensing systems expands when exposed to high temperatures. An expanded fill fluid occupies a larger volume in the cavities of the pressure transmitter and/or the capillary tube and cavities of the remote seal housing than that of a non-expanded fill fluid. A large volume of fill fluid affects the ability of the pressure sensor to accurately sense pressure of the process fluid as well as the ability of the isolation diaphragm to properly isolate the process fluid from the fill fluid. Many times the expansion of the fill fluid can ultimately degrade the systems in which pressure transmitters are utilized. One technique to address this is shown in U.S. Pat. No. 5,731,522 entitled “TRANSMITTER WITH ISOLATION ASSEMBLY FOR PRESSURE SENSOR” to Rosemount, Inc. of Eden Prairie, Minn., which is commonly assigned with the present application.